The use of sterile insects to control an insect population, such as that of the mosquito, requires millions of insects to be reared, sterilised and released. In order to release insects on the scale required the process of rearing has to be as automated as possible in order to be cost effective.
For some insect types there is no need to sort the insects on the basis of sex before they are released. However, for other insects it is essential to remove the females, for example, because they transmit disease or damage crops. As in mosquitoes the females transmit disease the sorting process for mosquitoes should be optimised to reduce to the minimum level possible the number of female mosquitoes released whilst minimising any loss of male mosquitoes.
Due to the differing life cycles of male and female mosquitoes two main stages of sorting are generally required for a release program. First, larvae need to be sorted from the pupae and then male pupae must be sorted from the female pupae. This may be achieved on the basis of differing sizes of larvae and pupae, for instance if male pupae and female pupae are different sizes (i.e. on average (mean) at the same time period in development). For example, in Ae. aegypti, larvae are smaller, of a different shape and thinner than male pupae. Moreover, pupae tend to float whilst larvae are prone to sink. Male pupae are smaller than female pupae.
In mosquitoes there have been several large scale rearing programs which have attempted to automate the rearing and separation of mosquitoes in their different life stages. Examples of known devices and methods which have been developed to automate sorting mosquitoes will now be described with reference to FIGS. 1 to 3.
One example of a sorting machine which was first described in Focks, D. (1980). “An improved separator for the developmental stages, sexes, and species of mosquitoes (Diptera: Culicidae).” J Med Entomol, 17, pp. 567-568 is illustrated in FIG. 1. The machine includes an aluminium plate which supports two glass plates. The distance between the glass plates is manually adjustable using 4 control knobs and is set to form a downward-pointing wedge-shaped space such that when aquatic insect culture is poured between the plates the larger specimens are captured higher up the plates and the smaller specimens are captured lower down the plates. However, in an experiment it was estimated that for An. albimanus the sorting efficiency of this machine was about 85% which meant that 15% of females were present after sorting. This is not sufficient efficiency to be of use in sterile insect techniques. In addition, use of the instrument is relatively labour-intensive as the spacing needs to be adjusted in use and therefore re-adjusted for each new batch of specimens to be sorted.
Another device is illustrated in FIG. 2 and is described by Evans F. D. S. and Evans H. T. in “A simple separator for mosquito larvae and pupae” in Mosquito News 28(4); 649-650. The device has three tubes, two placed in a horizontal plane with each other and being fixed. The third tube is placed underneath the two tubes and is moveable to provide an adjustable gap for sorting larvae from pupae.
In order to sort a sample the sample is poured between the two cylinders so that it flows over the third cylinder. Whilst the larger pupae are caught in the gap between the upper two cylinders and the lower third cylinder, the smaller larvae pass through the gap and can be collected. However, as the pupae get caught between the cylinders they impede the passage of the larvae and therefore the device cannot be used for high throughput sorting.
Cold water separation is a technique for separating larvae from pupae. In cold water separation larvae and pupae are immobilised in cold water (3-4° C.). At this temperature pupae float in water and larvae sink so the pupae and larvae automatically separate. The pupae can then be separated from the larvae, for example, by decanting or use of a vacuum picker. In experiments cold water separation has been found to cause damage and cold shock to the pupae. Additionally, the technique can only sort to around 80-90% efficiency meaning that the remaining larvae have to be sorted by hand. Furthermore, its efficiency is dependent upon the species of mosquito to be sorted.
Finally, the McCray sorter illustrated in FIG. 3 was developed to sort larvae from pupae. The sorter includes a plurality of slots present in a chamber. Larvae and pupae are poured into the device and a stream of water washes the larvae and pupae onto the slots. The larger female pupae are trapped by the slots but the male pupae and larvae wash through the slots and in this way the female pupae are separated from the male pupae and larvae. However, as with the device described by Evans and Evans the pupae trapped by the slots prevent impede the passage of larvae through the slots meaning the McCray sorter is not suitable for high throughput sorting as the pupae have to be removed from the slots periodically. Additionally, there is no method for removing the larvae from the male pupae.
There is, therefore, no device which appears to efficiently sort larvae from pupae and male pupae from female pupae.